The present invention relates to catheter tracking systems which serve to determine a position of catheters within the human or animal body. Furthermore, the present invention relates to methods for tracking catheters within the human or animal body.
The term catheter as used herein refers to any type of invasive surgical tool, used for insertion into a human or animal body for the purpose of providing remote access to a party of the body for performing some type of investigative and/or medical procedure.
With the increasing use of minimally invasive surgical techniques in medical diagnosis and therapy, there is a need for new methods of remotely locating and tracking catheters or other medical instruments inside a human or animal body. Currently, X-ray fluoroscopic imaging is the standard catheter tracking technique. However, excessive exposure to X-ray dosages by both the patient and clinician can be harmful. Thus, alternative catheter tracking methods are desirable.
Several alternative methods have been published including some which employ magnetic field measurements and others using ultrasonic measurements. One such ultrasonic catheter tracking technique is known as sonomicrometry. Sonomicrometry is based on finding distances between miniature omnidirectional ultrasound transducers by measuring a time taken for ultrasound signals to travel between the ultrasound transducers and then multiplying this by the speed of sound. It is assumed that the average speed of sound in the medium between the transducers is known and that the sound travels along a straight line. Both of these assumptions introduce errors into the distance calculations, ultimately leading to a level of uncertainty in the catheter location.
To locate the tip of a catheter using sonomicrometry, an ultrasound transducer is mounted proximate the catheter tip. A location of this transducer is then determined by measuring a time of flight of acoustic signals from the transducer on the tip to at least four other transducers acting as reference transducers disposed to detect the acoustic signals. The time of flight of the acoustic signals between the transducer on the tip and the reference transducers is representative of a distance of the tip of the catheter to the reference transducers. In combination, these distances serve to provide an indication of a position of the catheter in a three dimensional reference frame defined by the positions of the reference transducers.
A known catheter tracking system based on these sonomicrometric principles is described in U.S. Pat. No. 5,515,853 (Smith et al). This system measures the ultrasound travel times between pairs of transducers using short pulses of sound and clocked digital counters. The counters are started by the electrical pulse which drives the transmitting transducer, and are stopped by the detection of a pulse at the receiving transducer. Detection is accomplished by thresholding the received signal. Each transmitting transducer is activated in turn, after waiting for the last transmitted pulse to arrive at all receiving transducers, and for stray reflections from the various discontinuities inside the body to die away.
A disadvantage of this known catheter tracking system is that ultrasound signals do not travel in a straight line. Additionally, the speed of propagation of any ultrasound wave is dependent upon the material in which it is travelling. Ultrasound waves are subject to absorption, reflection, refraction, and scattering effects due to the material along its path, resulting in a loss of signal strength. An ultrasound wave travelling in the human body will suffer from all of the aforementioned effects, resulting in an error associated with each time of flight measurement, leading to uncertainty in determining the catheter location.
A technical problem of improving an accuracy with which a catheter tip is located is addressed by the catheter tracking system according to the present invention.